This application relates to wireless communication networks.
Wireless communication systems provide voice or data services to a plurality of wireless or mobile stations situated within a geographic region by dividing the region into a number of cells, conceptually represented by a hexagon in a honeycomb pattern. In practice, however, each cell may have an irregular shape, depending on various factors including the terrain surrounding the cell and traffic density. Each cell may be further divided into two or more sectors. Each cell contains system communication equipment such as a base station that transmits communication signals to the mobile stations on the forward link and receives communication signals from the mobile stations on the reverse link.
One exemplary wireless communication system designed for high speed packet data services is 1xEV-DO, which is also known as High Date Rate (HDR) or High Rate Packet Data (HRPD) system. 1xEV-DO has been standardized as C.S0024 in the international standard group Third Generation Project Partnership Two (3GPP2) and has been published as IS-856 Revision 0 and Revision A standards in the United States.
In 1xEV-DO system, a mobile station, which is also known as the access terminal or AT, determines and reports the data rate that can be supported on the forward link in the Data Rate Control (DRC) message. The base station, which is also known as the access network or AN, selects one Physical Layer packet for forward link transmission at a particular time slot, based on the DRC messages received from various mobile stations. The Physical Layer packet may be given more than one time slot for transmission. In this case, the transmit slots of a Physical Layer packet are separated by three intervening slots, during which the slots of other Physical Layer packets can be transmitted. If a positive acknowledgement (ACK) is received on the reverse link ACK Channel before all of the allocated slots have been transmitted, the remaining un-transmitted slots will not be transmitted and the next allocated slot may be used for the first slot of a new Physical Layer packet transmission. This technique is known as Hybrid Automatic Repeat Request (HARQ).
In a 1xEV-DO system, in order to identify the target mobile station of the forward data packet, the base station transmits a preamble on the I-branch, which is the in-phase branch of the complex signal, before the data packet. Meanwhile, no signals are transmitted on the Q-branch, which is the quadrature branch of the complex signal. The preamble contains the repetition of 32-chip bi-orthogonal sequence as in 15-856 Revision 0 standard, or repetition of 64-chip bi-orthogonal sequence as in 15-856 Revision A standard. The 32-chip bi-orthogonal sequence is defined in terms of the 32-ary Walsh functions and their bit-by-bit complements byWi/232 for i=0, 2, . . . , 62  (1) W(i-1)/232 for i=1, 3, . . . , 63  (2)where i=0, 1, . . . , 63 is the MACIndex value and Wi32 is the bit-by-bit complement of the 32-chip Walsh function of order i. The MACIndex is a number, which is assigned by the base station for identifying a mobile station in the system. Some MACIndex values are used as common values to all mobile stations for the purpose to identify the Control Channel, Broadcast, or Multi-User Packet transmissions. The 64-chip bi-orthogonal sequence is defined in terms of the 64-ary Walsh functions and their bit-by-bit complements byWi/264 i=0, 2, . . . , 126  (3) W(i-11)/264 for i=1, 3, . . . , 127  (4)where i=0, 1, . . . , 127 is the MACIndex value and Wi64 is the bit-by-bit complement of the 64-chip Walsh function of order i. The repetition of 32-chip bi-orthogonal sequence is a subset of the 64-chip bi-orthogonal sequence, as Walsh functions can be generated by means of the following recursive procedure:
                                          H            1                    =          0                ,                              H            2                    =                                                    0                                            0                                                                    0                                            1                                                    ,                              H            4                    =                                                    0                                            0                                            0                                            0                                                                    0                                            1                                            0                                            1                                                                    0                                            0                                            1                                            1                                                                    0                                            1                                            1                                            0                                                    ,                              H                          2              ⁢              N                                =                                                                      H                  N                                                                              H                  N                                                                                                      H                  N                                                                                                                                      ⁢                                                            H                      _                                        N                                                                                      ,                            (        5        )            where N is a power of 2, HN denotes the Hadamard sequences with a length of N, HN denotes the bit-by-bit complements of HN. Further, in IS-856 standard the Walsh functions are generated by mapping the binary bits (i.e. “0” or “1”) of Hadamard sequences to bi-polar symbols +1 or −1 as follows:WN=1−2HN  (6)where WN denotes the Walsh functions with a length of N. Therefore, IS-856 Revision A standard doubles the MACIndex numbers while supporting the legacy mobile stations that comply with the IS-856 Revision 0 standard in an IS-856 Revision A network. The length of the preamble is variable from 64 chips to 1024 chips, depending on the data packet format. A maximum of 128 MACIndex values can be supported in an IS-856 Revision A system.
As 1xEV-DO evolves to provide broadband services, particularly with a multi-carrier based solution, the system may need to support more than 128 mobile stations for each sector. The industry is currently investigating methods that can increase the MACIndex numbers while maintaining backward compatibility in such a way that the legacy mobile stations can be supported in the same upgraded system.